During invasive medical procedures employing a tissue penetrating medical instrument, such as a needle or a scalpel, there is a need to identify tissue structures in advance of the medical instrument so as to control the depth of penetration of the tip or cutting edge of the medical instrument. This may be done to ensure that the penetration is to the right depth, but usually more importantly, this is done to ensure that the medical instrument does not penetrate too deeply and thereby damage tissue structure unnecessarily.
During surgery important tissue is often damaged inadvertently. For example, biopsy patients often report a loss of sensation or motion control due to nerve damage sustained during surgery. It would be desirable to provide surgeons with a means to visualize better or measure the distance of the tip of a surgical device, such as a scalpel, to nerve tissue so that the surgeon can avoid penetrating tissue so deeply as to damage the nerve tissue.
Another current medical problem occurs during tests of certain body fluids or during localized injections, where a needle used to extract body fluids or inject medication must penetrate one body structure without penetrating a subsequent structure. For example, in performing a spinal tap it is desirable to penetrate the dural membrane containing the spinal fluid, but highly undesirable to penetrate, or even touch the spinal cord itself as that can cause sever injury and potentially permanent paralysis. Yet, this is generally carried out without the aid of any means for seeing the physical relationship of the tip of the spinal tap needle to the spinal structures or even a way of measuring the distance from the tip to structure that must not be penetrated. Similarly, in carotid injections in small laboratory animals it is often difficult to find the artery or vein in small animals, and it can be very difficult to keep from penetrating entirely through the artery or vein, once it is found. This can make drug delivery excessively time consuming and difficult to control.
The use of optical coherence domain reflectometry (“OCDR”) has been disclosed as a technique for examining the reflectivity of an animal structure to a limited depth therein. In OCDR a short coherence length light source is used in a scanning Michelson interferometer to determine the distance of a point from which light is scattered to a reference position. This is disclosed, for example, in Swanson et al. U.S. Pat. No. 5,459,579 entitled METHOD AND APPARTUS FOR PERFORMING OPTICAL MEASUREMENTS, hereby incorporated by reference in its entirety. In particular, Swanson et al. discloses the use of a fiber optic Michelson interferometer having a test fiber placed adjacent to the surface of an animal structure and coupled through a lens for illuminating the structure and coupling the backscattered light back into the fiber. The backscattered light is then interfered with a scanning reference reflector to measure the reflectivity profile of the structure to a limited depth therein. However, the usefulness of this technique is limited by the size and vulnerability of the lens assembly associated with the test fiber.
Tearney, et al., U.S. Pat. No. 6,134,003, entitled METHOD AND APPARATUS FOR PERFORMING OPTICAL MEASUREMENTS USING A FIBER OPTIC IMAGING GUIDE WIRE, CATHETER OR ENDOSCOPE, hereby incorporated by reference in its entirety, extends OCDR to optical coherence tomography (“OCT”). Tearney et al. discloses that an OCDR test fiber may be combined with a catheter or endoscope having a scanning imaging system at the distal end thereof for obtaining multiple measurements of the distance to a body structure used to create a tomograhic image of the structure. Like Swanson et al., the disclosure of Tearney et al. is limited by the use of an optical system at the distal end of the test fiber, and it is static with respect to tissue depth. Colston et al. U.S. Pat. No. 6,175,669 entitled OPTICAL COHERENCE DOMAIN REFLECTOMETRY GUIDEWIRE, which discloses a fiber optic OCDR system for guiding a guidewire through an arterial system is similarly limited.
A method for analyzing tissue based on the information obtained from OCT is also discussed in Song et al., “Simultaneous measurements of thickness and refractive index of microstructures in obscure specimens by optical coherence tomography,” Optik Volume 111, Issue 12, pages 541-543 (2000). Like Swanson et al. and Tearney et al., the disclosure of Song et al. is limited optics and essentially static depth measurements.
A technique for analyzing the contours of eye surfaces using OCT to provide autofocussing of a laser scalpel in eye surgery has been disclosed in Wei et al. U.S. Pat. No. 6,004,314 entitled OPTICAL COHERENCE TOMOGRAPHY ASSISTED SURGICAL APPARATUS. While in-line tomography, that is, obtaining OCDR data relative to various locations along the axis of propagation of the OCDR light is disclosed, this system is limited to use with a non physically invasive laser tool applied to the eye, which is especially adapted for light transmission, and is limited by the use of large, free-space optics for coupling the OCDR into the eye.
Winston et al. U.S. Pat. No. 6,228,076 entitled SYSTEM AND MTHOD FOR CONTROLLING TISSUE ABLATION describes the combining OCDR with an endoscope used for laser ablation to distinguish tissue. However, it is limited in applicability by the size of the endoscope and distal optics.
In both manual and robotic surgery, the surgeon and surgical robot must know: (a) where a cut is occurring, and (b) what is being cut. This information describes the current state of a procedure. In addition, ideally the surgeon and robot would know where a cut is about to occur and what is about to be cut. This information, referred to as feedforward, is much harder information to acquire.
A surgical robotic arm typically provides, from sensors mounted thereon, six position values, six velocity values, and six acceleration values. Combining this information with basic anatomy information can let the robot know where and what it is cutting and to estimate what it is cutting, but does not allow the robot actually to sense the tissue that is being cut or to sense ahead of the cut. Such advance, or feedforward, information, as well as feedback, has been supplied for surgical procedures by, magnetic resonance imaging (“MRI”) and ultrasound on a routine basis. However, neither of these techniques has sub-millimeter resolution. So, for delicate work they are not optimal. In addition, MRI is severely constrained in application by its size, its cost, and the strength of the magnets involved. Ultrasound has considerably worse spatial resolution than MRI.
Accordingly, there is a need for a method and system that can be used both to determine the current state of a mechanical tissue penetrating medical instrument, such as a mechanical scalpel, biopsy needle, or injection needle, to scan tissue ahead of the medical instrument while it is moving, to identify and avoid damaging tissue structures such as blood vessels and nerves.